Catalytic ozone decomposition and adsorptive VOCs removal in bimetallic metal-organic frameworks

Atmospheric ozone has long been a threat to human health, however, rational design of high-performance O3-decomposition catalysts remains challenging. Herein, we demonstrate the great potential of a series of isomorphous bimetallic MOFs denoted as PCN-250(Fe2M) (M = Co2+, Ni2+, Mn2+) in catalytic O3 decomposition. Particularly, PCN-250(Fe2Co) showed 100% O3 removal efficiency for a continuous air flow containing 1 ppm O3 over a wide humidity range (0 ‒ 80% RH) at room temperature. Mechanism studies suggested that the high catalytic performance originated from the introduction of open Co(II) sites as well as its porous structure. Additionally, at low pressures around 10 Pa, PCN-250(Fe2Co) exhibited high adsorption capacities (89 ‒ 241 mg g−1) for most VOCs, which are not only a class of hazardous air pollutants but also the precursor of O3. This work opens up a new avenue to develop advanced air purification materials for O3 and VOCs removal in one.


S3
nitrate was added into the aqueous solution of 2-methylimidazole under stirring at room temperature. 5 After stirring for 4 h, the product was collected by centrifuging, washed with deionized water (3 × 40 mL) for 48 h and methanol (3 × 40 mL) for 48 h, and then dried under vacuum at 80 °C for 8 h.

Computational details
It is commonly known that there are several different oxidation states available for both Fe and Co atoms, which can lead to distinct spin multiplicities for the PCN-250(Fe2Co) model ( Supplementary Fig. 1). Thus, we first need to decide the most stable spin state, from which the catalytic reaction starts to proceed. A series of DFT calculations show that it has the lowest energy when the total spin multiplicity is 14 (i.e. 13 unpaired electrons, see Supplementary Table 1). Electronic structure analysis shows that oxidation states of the Fe and Co atoms are +3 and +2 in the cluster model of PCN-250(Fe2Co), respectively. This means that 10 unpaired electrons singly occupy 10 3d orbitals of the two Fe(III) atoms and 3 unpaired electrons singly occupy 3 3d orbitals of the Co(II) atom. In such situation, there are two doubly occupied 3d orbitals of the Co(II) atom. Interestingly, all unpaired electrons are mainly located on metal atoms and do not involve ligands, which is supported by the calculated spin density plotted in Starting from the ground spin state of S = 14, we have separately calculated corresponding catalytic reaction paths of O3 decomposition. First, we focus on the reaction mechanism under humid condition. In such case, a water molecule first occupies the Co(II) atom, this process is favorable thermodynamically with an adsorption energy of 22.4 kcal mol −1 . Moreover, the water adsorption does not change the lowest spin state (still S = 14). As shown in Fig. 4a and 4c in the maintext, starting this H2O-coordinated PCN-250(Fe2Co) complex, the first step is a hydrogen transfer reaction process from H2O * to O3, i.e. from REACT1 to INT1-1, which produces • OOOH and *• OH ( * stands for the atom that is coordinated with the Co atom). It requires 15.9 kcal mol −1 energy to overcome the energy barrier at TS1-1 in the lowest spin state with a total spin multiplicity of 14 (S = 14). Although this process becomes easier in the spin state with S = 16, this path is unimportant because of much higher energy of reactant. Interestingly, there is a crossing point between the two spin states (S = 14 and 16) in the vicinity of INT1-1 and their energies are calculated to be 11.8 and 13.8 kcal mol −1 , respectively. Thus, near INT1-1, the system will hop to the spin state with S = 16 via an intersystem crossing process. Subsequently, further hydrogen transfer from *• OH to • OOOH to produce O2, H2O, and • O * takes place by overcoming a barrier of 2.5 kcal mol −1 . Here, the reason for the final formation of • O * is that an electron of Co is bonded with an electron of • O * . The bond order is 1.16 and the bond length is 1.62 Å.
Finally, a new O3 molecule attacks the • O * atom coordinated with the Co atom generating two triplet O2 molecules, which demands 6.6 kcal mol −1 energy to overcome the barrier at TS1-3 leading to a complex of PCN-250(Fe2Co) and two triplet O2 molecules. Once these two O2 molecules leave away, the original PCN-250(Fe2Co) catalyst is recovered with the lowest spin state of S = 14. Therefore, this step must involve a spin flip process after the system overcomes the barrier at TS1-3. To sum up, the rate-determining step is the first hydrogen transfer reaction in the spin state of S = 14 with a barrier of 15.9 kcal mol −1 ; the entire catalytic reaction is allowed thermodynamically because of releasing 73.8 kcal mol −1 energy; importantly, the reaction involves two spin states and takes place in a nonadiabatic means.

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Second, under dry condition, O3 is directly adsorbed on the exposed Co(II) atom with an adsorption energy of 9.0 kcal mol −1 . Note that the spin state of S = 14 is still the lowest upon O3 adsorption. As shown in Fig. 4b and 4d in the maintext, the first step involves the O-O bond fission of * O3 coordinated with the Co atom producing one triplet O2 molecule and • O * ; thus, it must encounter a spin-flip process. The corresponding spin crossing point is determined and its energy is 9. 4  Under humid condition, all three metal centers are coordinated with water at initial state ( Fig. 4c in the maintext), but under dry condition, only Co center is uncoordinated ( Fig. 4d in the maintext). The construction of such an uncoordinated model was based on the following considerations. Under dry condition, the Co(II) and Fe(III) centers are all coordinated with water at the very beginning. The O3 decomposition reaction can happen on either the H2O-coordinated Co(II) sites or the H2O-coordinated Fe(III) sites.
In the former case, the reaction proceeds following the pathway shown in Fig. 4a in the maintext, producing a coordinatively unsaturated Co(II) site (State 5). The energy barrier of the rate-determining step is calculated to be 15.9 kcal mol −1 (Fig. 4c in the maintext). In the latter case, the O3 decomposition reaction proceeds in a similar way, but the energy barrier of the rate-determining step (21.5 kcal mol −1 , Fig. 4e in the maintext) is higher than that in the former case. Therefore, the formation of a cluster S6 with one coordinatively unsaturated Co(II) site and two H2O-coordinated Fe(III) sites is energetically favorable. After the coordinatively unsaturated Co(II) site is formed, the O3 decomposition reaction would happen on the open Co(II) site following the reaction pathway shown in Fig. 4b in the maintext, because the energy barrier of the rate-determining step becomes much lower (9.4 kcal mol −1 , Fig. 4d in the maintext). In short, under dry condition the starting model is set to contain one open Co(II) site and two H2O-coordinated Fe(III) sites by considering that the formation of such a model is more energetically favorable than the formation of other possible models.  decomposition test. The total inlet air flow rate was set to be 1L min −1 .

Reactor
Humidity generator with MFC    p-xylene exposed m-xylene exposed o-xylene exposed ethylbenzene exposed toluene exposed cyclohexane exposed n-hexane exposed benzene exposed acetone exposed 2 Theta (°)